I've only known 2 Micah's---one was a fellow student in junior high and I have no idea what his religion/background was.
The other was another baby in a mom/baby group I was in when my oldest (now age 12) was a baby. This Micah had a German immigrant mother and an American father (Army) and they were both Christian, not Jewish.
I would never think of Micah as a girl name, but it's not really one that I like. I always immediately think of Mica
http://en.wikipedia.org/wiki/Mica